The striatum is the primary input to the basal ganglia, a collection of structures involved in initiation of voluntary movements and motor learning. This structure is involved in many movement and cognitive disorders. The principle output of striatum is through the GABAergic medium spiny neurons (MSNs). Cholinergic interneurons (CINs) of the striatum fire tonically and provide the sole source of the neuromodulator acetylcholine (Ach) to the striatum. In vitro, Ach presynaptically inhibits corticostriatal EPSCs in MSNs. This provides a powerful opportunity for regulation of MSN activity. However, it has been difficult to assess the role of CIN firing on this inhibition. Furthermore, CINs are thought to correspond to tonically active neurons (TANs) reported in vivo. These cells briefly cease firing to rewarding stimuli and stimuli that predict reward, suggesting the possibility that this pause reduces Ach relieving this inhibition. In this project we seek to link the activity of CINs to changes in MSN firing and corticostriatal EPSCs both in vitro and in vivo. We selectively control the firing of CINs using virally delivered light activated membrane proteins, channelrhodopsin (ChR2) and halorhodopsin (HR). We have validated that we can control the firing of CINs in vivo and in vitro. We can now investigate how both the timing of CIN action potential and their firing rate affects EPSCs in MSNs in the acute slice. And, with HR we now have the ability to briefly pause CINs in striatum. We will observe any changes to corticostriatal EPSCs, allowing us to make predictions on the function of TAN pauses. Additionally, using implanted fiberoptics and in vivo recording devices we can activate and pause CINs in vivo. By comparing evocable CINs to apparent TANs we can identify if CINs are in fact TANs. And by recording the activity of the surrounding MSNs we can identify the net effect of CIN activity or pause on spontaneous and cortically driven activity. This approach allows the first opportunity to observe the effects of cholinergic interneuron activity, and provides the first insight to a function of the behaviorally important pause in TAN firing.